The present invention relates to electronic circuits, and more particularly, to cascode circuits.
The cascode is a circuit configuration that has numerous applications. A transistor may have a small output resistance in an application that requires a large output resistance. Adding a cascode transistor can boost the output resistance.
FIG. 1 illustrates an example of a conventional cascode circuit 100. Cascode circuit 100 includes n-channel metal oxide semiconductor field-effect transistors (MOSFETs) 102 and 104 and p-channel MOSFETs 106 and 108. Transistor 102 receives an input voltage VIN at its gate. Cascode transistor 104 receives a bias voltage VBIAS at its gate. The gate and the drain of cascode transistor 106 are coupled together and to the drain of transistor 104. The gate of transistor 108 is coupled to the gate and the drain of cascode transistor 106.
Some types of cascode circuits can be used to implement current-sources. An ideal current-source generates a constant current, independently of the output voltage of the current-source.
However, impact ionization current in MOSFETs adds to the drain current at high drain-to-source voltages. Electrons drift from drain to source in an n-channel MOSFET. When the electric field across a MOSFET reaches a critical electric field, the drift velocity saturates. Above the critical electric field, hot carriers can cause impact ionization. Impact ionization can result in current flow from the channel to the substrate of a MOSFET. The channel-to-substrate current flow adds to the drain current. The extra drain current is undesirable, because it can cause the current of a current-source to vary.
The current generated by a current-source can vary if the supply voltage to the current-source is increased above a limited voltage range. Therefore, it would be desirable to provide a current-source circuit that generates a constant current across a wider supply voltage range.